One approach to the melt-lamination of thermoplastic materials is to use an extrusion apparatus including a combining adaptor, a die and a connecting duct, which in cross-section is typically rectangular in shape. In the adaptor, flow channels converge as a result of which there is a confluence of the molten resin streams that flow through the flow channels, to form a melt-laminate. The layered melt stream exits from the combining adaptor, passes through the connecting duct and flows into the die, which is typically a single manifold die. In the die manifold, the melt-laminate diverges at a locus of divergence and transverse flow takes place over the length of the manifold. Hence, the layered melt stream leaving the manifold is wider than the stream entering the manifold. This wider melt-laminate flows through an exit channel and exits from the die.
A problem with this type of apparatus is that a laminate formed from thermoplastic materials of dissimilar flow properties will have layers of non-uniform thickness. A. A. Khan and C. D. Han, Transactions of the Society of Rheology, vol. 20(4), pp. 595-621 (1976) is illustrative of investigative studies relating to this problem. FIG. 7 of this publication shows that the layers of a melt-laminate become increasingly non-uniform as the melt-laminate progressively flows through a rectangular duct. Furthermore, passing the layered melt stream through a die manifold additionally promotes layer non-uniformity.
A further problem with this type of apparatus is that each layer of the laminate formed thereby exhibits the curtaining effect. The curtaining effect is a well known defect in the layers of a thermoplastic laminate. It is characterized by a pattern in the individual layers that is particularly apparent when the layers are pigmented.
An extrusion apparatus is known in which the combining adaptor is connected directly to a die. This type of apparatus, which is exemplified by U.S. Pat. No. 3,761,211 to Parkinson, advantageously lacks the connecting duct. However, it does not provide a solution to either of these problems.
Also known in the prior art, as illustrated by my U.S. Pat. Nos. 4,152,387 and 4,197,069, is a combining adaptor having an adjustable divider provided between any two of the flow channels thereof. Each flow channel includes a back pressure cavity and a flow restriction channel located between the back pressure cavity and the point of convergence of the flow channels. This combining adaptor provides for adjustment of flow restriction channel width by manipulation of the adjustable divider. The layers of a laminate made by use of this adaptor in combination with a conventional single manifold die, exhibit the curtaining effect.
One solution to the problem of layer non-uniformity has been to profile the melt-laminate, that is, to transform the cross-sectional configuration of each stream of the melt-laminate from a rectangular configuration to a shape that is changed back to the original rectangular configuration as the melt-laminate passes from the point at which it is formed, through the die manifold. Profiling has been found to produce a laminate with layers each of uniform thickness, from thermoplastic materials of dissimilar flow properties. However, a drawback with profiling is that experimentation is usually required to determine the precise shape to which it is necessary to intentionally alter the cross-sectional configuration of the layered melt stream. Experimentation is, of course, expensive in terms of both time and money.
From the preceding discussion, it is clear that there is a need for an extrusion apparatus that includes a combining adaptor and a die, that will produce a laminate with layers each of substantially uniform thickness from thermoplastic materials of dissimilar flow properties without profiling of the melt-laminate formed from these thermoplastic materials. Such an improved apparatus would be especially remarkable if it were capable of handling resins of varying viscosities, merely by removal of and replacement of a component thereof with an interchangeable component exactly configured for specific resin viscosities. Moreover, such an improved apparatus would provide an even greater contribution to the art if it lessened the curtaining effect. Such an apparatus would make possible an improved process for the melt-lamination of thermoplastic materials.